1. Field of the Invention
The present invention relates to reproducing apparatus for a video disc and, more particularly, is directed to a reproducing apparatus for a video disc for eliminating spurious components included in a video signal reproduced from the video disc.
2. Description of the Prior Art
Conventionally, as shown in FIG. 1, on an optical video disc, there are recorded in frequency multiplication a frequency modulated video signal VFM having a sync-tip frequency of 7.6 MHz and a white peak frequency of 9.3 MHz, and a frequency modulated audio signal AFM of two channels (right and left channels). Carrier frequencies of the two channel frequency modulated audio signals AFM are 2.3 MHz and 2.8 MHz, respectively.
Upon recording, the frequency modulated video signal of a shorter period shown in FIG. 2A and the frequency modulated audio signal of a longer period shown in FIG. 2B are added to form a composite signal shown in FIG. 2C. The composite signal is limited in its amplitude to thereby be converted into a recording signal as shown in FIG. 2D. In the recording signal, a change in a repetition frequency thereof represents a frequency modulated component of the video signal and a change in a pulse duty factor thereof represents a frequency modulated component of the audio signal.
In the optical video disc, the frequency modulated video signal VFM and the frequency modulated audio signal AFM are recorded in frequency multiplication as set forth above. Thus, upon reproduction, due to inter-modulation of the frequency modulated video and audio signals, there are generated many spurious components including a difference mixing component and a sum mixing component between a carrier frequency fv of the frequency modulated video signal VFM and a carrier frequency fa of the frequency modulated audio signal AFM. Thus, the conventional reproducing apparatus for an optical disc has the disadvantage that beat-shaped noises appear in a reproduced image due to the spurious components of a demodulated video signal having frequency components almost equal to the carrier frequency of the frequency modulated audio signal AFM, thereby degrading quality of the reproduced video image.
One method of eliminating the spurious noises has been proposed by the same assignee of the present application in Japanese Laid-Open Publication No. 61-199271. In this method, synchronous detection is performed based on a carrier frequency component (spurious component) of a frequency modulated audio signal included in a demodulated video signal and a carrier frequency component of a reproduced frequency modulated audio signal. Then, a level of the carrier frequency component of the reproduced frequency modulated audio signal is adjusted by an output of the synchronous detection, thereby the spurious components of the demodulated video signal being eliminated.
FIG. 4 shows an arrangement of the conventional reproducing apparatus for a video disc shown in the above-described publication.
Referring to FIG. 4, a reproduced radio frequency (RF) signal is outputted from an optical pickup (PU) 1 positioned in opposition to a video disc D, and then supplied to a video signal demodulator 12 through a band pass filter (BPF) 11 having a center frequency fv. Also, the reproduced RF signal is supplied to a pair of audio signal demodulators 15 and 16 through a pair of band pass filters 13 and 14 having center frequencies fa1 and fa2, respectively. Thus, an audio signal and a video signal are reproduced by the demodulators 15, 16 and 12, respectively. An output of the video signal demodulator 12 is, on one hand, supplied to an adder 19 through a low pass filter (LPF) 17 and a delay circuit 18 having a delay time T.
The output signal of the video signal demodulator 12 is, on the other hand, supplied to spurious correction circuits 20 and 30. In the spurious correction circuit 20, the output signal of the video signal demodulator 12 is supplied to a synchronous detection circuit 21 through a band pass filter 22 having a center frequency fa1. Also, a reproduced audio signal Sa1 is supplied from the band pass filter 13 to the synchronous detection circuit 21 through a delay circuit 23 having a delay time Ta1.
An output signal of the delay circuit 23 is also supplied to a level adjusting circuit 24. An output signal of the synchronous detection circuit 21 is supplied through a low pass filter 25 to the level detection circuit 24, which thereby controls an output signal level of the delay circuit 23. The signal thus adjusted in its signal level by the level adjusting circuit 24 is supplied to the adder 19 through a phase conversion circuit 26.
In the other spurious correction circuit 30, in the same manner as the spurious correction circuit 20, the output signal of the video signal demodulator 12 is supplied to a synchronous detection circuit 31 through a band pass filter 32 having a center frequency fa2. An output signal of the synchronous detection circuit 31 is supplied to a level adjusting circuit 34 through a low pass filter 35. The reproduced audio signal is supplied to a delay circuit 33 having a delay time Ta2 from the band pass filter 14. An output signal of the delay circuit 33 is supplied to the synchronous detection circuit 31 to perform the synchronous detection, and also supplied to the level adjusting circuit 34, which in turn controls an output signal level of the delay circuit 33 in accordance with an output of the low pass filter 35. An output signal of the level adjusting circuit 34 is supplied to the adder 19 through a phase inversion circuit 36.
In the synchronous detection circuits 21 and 31 of the spurious correction circuits 20 and 30, spurious components Ss1 and Ss2 having center frequencies fa1 and fa2 which are separated from the demodulated video signal are multiplied by audio carrier components Sa1 and Sa2 which are made in phase with the spurious components Ss1 and Ss2 by the delay circuits 23 and 33, respectively. Thus, the synchronous detection circuits 21 and 31 deliver detection signals Sd1 and Sd2 accorded with the spurious components Ss1 and Ss2, respectively.
Signal levels of the audio carrier components Sa1 and Sa2 are adjusted in accordance with the detection signals Sd1 and Sd2 by the level adjusting circuits 24 and 34, respectively. Thus, the level adjusting circuits 24 and 34 deliver correction signals Sc1 and Sc2 accorded with signal levels of the spurious components Ss1 and Ss2, respectively.
The correction signals Sc1 and Sc2 are inverted in their phases by the phase inversion circuits 26 and 36 and then supplied to the adder 19, respectively, whereby the spurious components Ss1 and Ss2 included in the demodulated video signal are eliminated in the adder 19.
In the thus constituted conventional reproducing apparatus for a video disc, since the spurious correction circuits 20 and 30 employ synchronous detection circuits 21 and 31, it is required to coincide the phases of the audio carrier components Sa1 and Sa2 for the synchronous detection with those of the spurious components Ss1 and Ss2 by using the delay circuits 23 and 33, respectively.
However, since the spurious component of the demodulated video signal differs at every disc, the conventional reproducing apparatus for a video disc has the disadvantage such that the delay times Ta1 and Ta2 of the spurious correction circuits 20 and 30 must be adjusted at every disc, respectively.